The present invention relates to a method for reducing the working temperature of a tire tread for vehicles and some tire treads capable of performing said method.
More in particular, the present invention relates to a method for reducing the working temperature of a tire tread for vehicles utilizing a tire tread consisting of a first and a second portion made of an elastomeric mixture comprising a filler, wherein the filler present in said first portion is mainly composed of carbon black while a substantial part of the filler present in said second portion is composed of silica.
Modern tires are required to show not only a good performance when running on any type of surface, dry, wet, snowy, but also a good qualitative level referred to other functional features such as, for example, comfort, smoothness and mileage, and these features have to be maintained even when the tire runs under critical or extreme conditions. For example, a too high speed compared to the type of road, especially when combined with a high ambient temperature.
The performance of a tire depends on circumferential and/or transversal grooves and notches in the tread that form seams and/or blocks, variously arranged so as to form a specific tread pattern designed in view of the type of use expected for the tire. Typically, a tread pattern is said to be symmetrical when its outlook is the same irrespective from the rolling direction of the tire. By converse, it is said to be asymmetrical when its outlook changes depending on the rolling direction. Furthermore, a tread pattern is said to be directional when it identifies a particular, preferred direction of rotation of the tire, that is when it is specular with respect to the equatorial plane of the same.
Furthermore the performance of a tire is affected to a considerable extent by the working temperature. One of the most difficult problems is to reconcile a good resistance to wear with an adequate road-holding both at normal (i.e. 30xc2x0 C.-70xc2x0 C., generally 30-60xc2x0 C., typically 30-50xc2x0 C.) and high (i.e. more than 70xc2x0 C., and even more than 100xc2x0 C. in case of a very fast car) working temperatures, the latter being caused by the high thermomechanical stresses produced by driving in extreme conditions.
In fact, in order to achieve a good resistance to wear and low resistance to rolling it is necessary to use elastomeric mixtures showing a low level of working temperature. By converse, in order to achieve a good road-holding, it is necessary to use elastomeric mixtures having a high level of working temperature thereby dissipating a high amount of energy and ensuring high adherence of the tread to the ground.
Hence, the theoretical models for an optimal tread require conflicting working temperature levels.
So far, the attempts made in the art to improve the performance of a tire in a temperature range other than that of normal usage (i.e. of from about 10 to about 40xc2x0 C. in winter season and of from about 30 to about 60xc2x0 C. in summer season) resulted either in a substantial and undesired loss of road-holding and/or in a substantial worsening of the resistance to abrasion of the tread.
Now, it has been unexpectedly found a valuable solution to the above mentioned problem.
Initially the applicant faced the problem of producing tires having some colored portions, and more particularly tires whose tread includes at least one colored insert. In fact, a two-colored tire, in addition to exhibiting a pleasant and unusual aesthetic appearance, allowing for example the customization of a vehicle with preferred colors, would also have some practical advantages. For example, a first advantage would be to make the correct assembly of a tire easier on the vehicle in case of asymmetrical tires. A second advantage, would be to allow an immediate identification of each type of many tires stored in a warehouse.
The applicant departed from the consideration that the production of colored elastomeric mixtures precludes the addition, even in a minimal proportion, of carbon black as a filler for the elastomeric mixture since the coloring power of carbon black is so high that it conceal any other colored pigment.
Only the so-called white charges can, in fact, be colored with suitable pigments. Typical white charges for use in tires, in particular in the tread, are made of silicon compounds, preferably silica.
However, silica is known to provide an elastomeric mixture for a tire having a low resistance to rolling and a good road-holding on snowy and wet roads while reduces both road-holding on dry roads in highly severe conditions and resistance to wear. Hence, the use of silica as a filler for the elastomeric mixture in tire tread implies careful evaluations.
By examining the road test results from a number of tire prototypes having the tread made of two different circumferential and axially side-by-side portions, the first made of an elastomeric mixture charged mainly with carbon black (black elastomeric mixture) the second made of an elastomeric mixture charged only with a white charge (white elastomeric mixture), the inventors have unexpectedly perceived that the performances on a dry road of this tire are superior compared to both a first comparison tire wherein the very same tread is made from the very same elastomeric mixture charged with carbon black only and a second comparison tire wherein the very same tread is made from the very same elastomeric mixture charged with silica only.
In order to understand the reason for this unexpected and inexplicable effect, the applicant has also performed thermographies on the tread of the tires under test and has unexpectedly found that the two-colored tires always ran at a colder temperature than the comparison black tires. Even more unexpectedly, also the black portion per se of said prototypes leant to work at a lower temperature compared to the corresponding tread portion in the comparison black tires.
Without intending to be bound in any way by this theory, it has been assumed that the portion of white elastomeric mixture develops a synergic action that keeps cooler than usual the adjacent portion of black elastomeric mixture as well, thereby reducing the working temperature of all the tire in its entirety.
EP-A-0 658 452 discloses a tire wherein the tread band has a low content of carbon black and a high content of silicon (or other non conductive fillers) thereby being antistatic in nature. In order to increase the electric conductivity thereof, said tire further comprises at least one conductive insert made of an elastomeric material incorporated in the tread band and extending through the whole tread band thickness, being then exposed externally of the rolling surface, wherein the elastomeric material forming said conductive insert exhibits an electric resistivity lower than that of the elastomeric material forming the tread band. When said inserts are two or more their overall width measured parallel of the tire axis is of from 4 to 50 mm. Since the total width of a tread band ranges from about 135 mm to about 285 mm, the maximum width of the conductive inserts is of 37% of the width of the tread band.
Similarly, EP-A-0 681 931 discloses a tire tread which is electrically conducting. More particularly, this document teaches to make an electrically conducting tire tread comprising an elongated strip of tire tread compound of high resistivity having a transverse width (TR) and, within the transverse width (TR), a longitudinally extending conducting strip of low resistivity tire compound with a volume resistivity less than 108 ohm cm, said conducting strip extending from the top to the bottom surface of the tread strip. In a specific embodiment, the high resistivity compound comprises silica as reinforcement or filler material while the low resistivity compound comprises carbon black as reinforcing filler. The width of the conducting strip is preferably in the range of 5-25% of the width of the tire tread (TR). The working temperature of said tire tread is never mentioned.
In a first aspect, this invention refers to a method for reducing the working temperature of a tire tread for vehicles utilizing a tire tread consisting of:
a) a first portion (A) comprising:
100 parts by weight of an elastomeric material,
40-120 parts by weight of a filler comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and
3-40 parts by weight of at least a conventional additive;
b) a second portion (B) comprising:
100 parts by weight of an elastomeric material,
40-120 parts by weight of a filler comprising from 30 to 100% by weight of silica and from 0 to 70% of carbon black, and
3-40 parts by weight of at least a conventional additive, provided, however, that the silica content in portion (B) is at least 20% higher than in portion (A).
In the present description and in the claims attached thereto, the terms xe2x80x9cportion Axe2x80x9d and xe2x80x9cportion Bxe2x80x9d do not imply that the tire tread of this invention is made of a single strip xe2x80x9cAxe2x80x9d and a single strip xe2x80x9cBxe2x80x9d. A number of strips xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d may be variously combined in the tire tread of this invention such as, for example, A/B, B/A, A/B/A, A/B/A/B/A, and so like. Any strip xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d will have, however the composition set forth above.
Although said portions A and B will be preferably arranged circumferentially (i.e. longitudinally), they may also be arranged transversally, either perpendicularly or diagonally, with respect with the equatorial plane of the tire.
Typically, the amount of filler in said first portion (A) is of from 60 to 80 parts by weight.
Preferably, the percentage of carbon black in said filler of said first portion (A) is of from 80 to 100% by weight.
Typically, the amount of filler in said second portion (B) is of from 60 to 80 parts by weight.
Preferably, the percentage of silica in said filler of said second portion (B) is of from 60 to 100% by weight.
More preferably, the percentage of silica in said filler of said second portion (B) is of from 80 to 100% by weight.
Even more preferably, the percentage of silica in said filler of said second portion (B) is of 100% by weight. In such a case, the elastomeric mixture of portion (B) preferably comprises also at least one coloring pigment selected from the well-known pigments usually added to the elastomeric mixtures.
Typically, said elastomeric material is selected from natural rubber, 1,4-cis polybutadiene, polychloroprene, 1,4-cis polyisoprene, optionally halogenated isoprene/isobutene copolymers, butadiene-acrylonitrile rubber, styrene-butadiene rubber, styrene-butadiene-isoprene terpolymers prepared either by solution or emulsion polymerization, ethylene-propylene-diene terpolymers, and mixtures thereof.
Preferably, said conventional additives are selected from vulcanizers, vulcanization accelerating agents, ZnO, stearic acid, antioxidants, plasticizers, antiaging agents, antifatigue agents, gypsum, talc, kaolin, bentonite, TiO2, silicates, colored pigments, binding agents capable of chemically reacting with silica in order to bind the latter to the elastomeric mixture during vulcanization, and mixtures thereof. A typical example of binding agent is the silanic compound Si69 by Degussa, Germany.
The tires where the area of said portion A exceeds 37% of the total area of the tire tread are in the tires of the present invention, the area of said portion A exceeds 37% of the total area of the tire tread.
For the purpose of this invention, the expression xe2x80x9ctotal area of the tire treadxe2x80x9d is intended to mean the circumferential portion of the tire whose width (L in FIGS. 1,3 and 5, circle arch Cxe2x80x94C in FIG. 2) is delimited by the two lines running where the tread intersects the two sidewalls of the tire.
In a second aspect, this invention refers to a tire for vehicles having a tread consisting of:
a) a first portion (A) comprising:
100 parts by weight of an elastomeric material,
40-120 parts by weight of a filler comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and
3-40 parts by weight of at least a conventional additive;
b) a second portion (B) comprising:
100 parts by weight of an elastomeric material,
40-120 parts by weight of a filler comprising from 30 to 100% by weight of silica and from 0 to 70% of carbon black, and
3-40 parts by weight of at least a conventional additive, wherein the silica content in portion (B) is at least 20% higher than in portion (A).
characterized in that the area of said portion A is of from 37.1% to 80% of the total area of the tire tread.
Preferably, the area of said portion A is of from 40% to 70%, and even more preferably of from 50% to 60% of the total area of the tire tread.